Background: The criteria for evaluating the degree of severity of dependence has an impact on estimating the costs of the long-term care services, especially among the population over 65 years of age. The sensitivity of the dependency rate must be analyzed in view of different definitions of seriousness, whether by the maximum severity observed in the disability in activities of daily living or by the number and severity of all these disabilities. Methods: The data from the Disability, Deficiency and Health Condition Survey conducted by the Spanish National Institute of Statistics in 1999 are used. A comparison is drawn between two criteria for evaluating the severity of an individual's disabilities. Models are made with the logarithms of the prevalence rates by age, for each sex, a comparison of the existence of significant differences between these criteria being drawn based on 95% confidence intervals. Results: The criterion based on the maximum severity (employed by the Spanish National Institute of Statistics) underestimates the degree of total dependency measured by the alternative criterion by 10%-25%, overestimating the degrees of moderate dependency as of age 65 by up to 25%. Among the male population, this same criterion gives rise the medium dependency as of age 95 being underestimated by 10%. Conclusions: The maximum severity criterion employed by the Spanish National Institute of Statistics underestimates the most serious dependency rate, especially as of 80 years of age.